Variable condenser



Nov. 7, 1939. R. c. SPRAGUE VARIABLE CONDENSER Filed Dec. 9, 1937 0 s 5,R n mw w Mmvm i 0 51 MW m a Patented Nov. 7, i939 EENETED to SpragueSpecialties Co,

North Adams,

Mass, a corporation of Massachusetts Application December 9, 1937,Serial No. 178,987

5 Claims.

The present invention relates to variable con-- densers, and will bedescribed in connection with the type commonly known as trimmer orpadding condensers which find their chief application as 5semi-adjustable means for tuning the resonant circuits of radioreceivers to fixed selected frequencies.

Such condensers usually comprise as their main elements at least onefixed and at least-one flexible and/or adjustable metal plate orrmature, between which metal plates is disposed a layer of dielectricmaterial, for example, of mica, the assembly being mounted on a suitablebase of lowloss insulating material. To obtain a continuous variation ofthe capacity, the flexible plate is made of a resilient material havinga high degree of elasticity, and its capacitative relationship to thefixed plate is varied by flexing it towards or away from the fixed plateby means of an adjustment screw or the like.

The main difficulty heretofore encountered in prior art condensers wasthat they were incapable of maintaining their adjusted capacity value,mainly because of the temperature changes and subjected in operation.

Such influences, as just stated, afiect the capacity of the condenser bychanging the slicetive area of the plates, the eiiective distance beatween the plates, and the efiective dielectric constant of thedielectric.

There have been various suggestions to overcome these difiicultieseither by constructions preventing changes in the adjusted capacity ofthe condenser or by auxiliary means which would compensate for suchchanges. Such solutions were only partly successful, i. e., only to ex-:

tent that they eliminated or compensated for capacity changes which wereof a regular and predictable nature.

I have found, however, that in practice capacity changes take place inthe operation of such condensers which are highly irregular and or anerratic and unpredictable nature and the lidfluence of which has notbeen and could not be overcome in prior art constructions,

My invention, therefore, relates to adjustable condensers particularlyof the so-called trimmer type, in which'the possibility oi suchunpredictable capacity changes is eliminated. The occurrence of suchunpredictable capacity changes can be explained as follows:

In the assembled condenser the fixed electrode plate is intended tocontact with its entire efiiec vibration to which the condensers arenormally tive surface with the dielectric layer, whereas the fiexible oradjustable plate contacts therewith with a smaller or larger surfacedepending upon the adjustment of the condenser.

An air gap defined by the flexible plate and the dielectriclayer-normally consisting of a mica sheetforms a second dielectric inseries with the mica, and the adjustment of the condenser takes place byvarying the average thickness of the air gap.

lhat the condenser for a given adjustment assumes adefinite capacityvalue and that after passing through a heat cycle returns to such value,is predicated among other things on the assumptions that the contactbetween the electrode plates and the dielectric layer is perfect andthat the air gap for a given setting is reliably defined by theflexible, plate and the dielectric layer.

With the prior art constructions these assumptions are found not to becorrect. While the ad'- justing means tends to press the dielectriclayer against the fixed plate, the contact between the twois far fromperfect. llhis is'due to the facts that the fixed condenser plate andthe dielectric layer consist of materials which are not plastic atordinary temperatures, and that the surfaces oi these elements andparticularly that of the metal plate are microscopically rough. @neresult thereof is that an air film of indefinite and non-uniformthickness is formed between the fixed plate and the contactingdielectric layer.

In operation this air film changes its effective area and averagethickness for various reasons. For example, when the condenser heats upin operation, the electrode plate and the dielectric layer, because oftheir different expansions, are relatively displaced,

Again when. cooling down, these elements contract and tend to assumetheir former relative position; however, the friction between theircontacting surfaces interferes with this.

Furthermore, at the higher operating temperatures the materials of thecondenser elements hecome increasingly plastic, and have a tendency toflow into each other, and as a rule the condensers after having passed anumber of heat cycles are found to have increased in capacity.

From the above it will be noted that the imperfect contact between thedielectric layer and the fixed electrode plate, and the variations inthe extent efiective thickness of the air film between same, causecapacity variations of the condenser, which are of an erratic nature andwhich have not been taken care of in prior art constructions.

To eliminate the capacity variations due to the imperfect contact andthe air film between the dielectric layer and the fixed condenserelectrode, I provide between the two 'a perfect molecular contact eitherby applying to the fixed plate a thin firmly-adhering dielectric layer,or preferably by forming the fixed condenser plate as a thin metalliclayer firmly adhering to the dielectric layer. The latter may beachieved by various well-known methods, for example, by electroplating,painting or spraying the metal-in a finely divided state and held by asuitable binder onto the dielectric layer.

While the above measure removes partly the causes responsible for theerratic capacity variations found in such condensers, it did noteliminate them altogether.

More particularly, I found that a condenser in which the fixed electrodewas formed as a metallic layer adhering to the dielectric gavesatisfactory results as long as the percentage capacity adjustment wassmall; i. e., as long as the flexible electrode contacted only with asmall portion'of the dielectric layer. However, at large percentagecapacity adjustments the behavior of the condenser was not much moresatisfactory than that of condensers of the prior art.

I found that this was due to an imperfect contact also occurring betweenthe flexible plate and the dielectric layer and an air film also formingbetween their contacting surfaces, the effect of which, however,manifests itself only with large percentage capacity setting.

' This can be explained as follows: As long as the capacity adjustmentis small, the flexible electrode plate contacts with the dielectriclayer over a very small surface. Consequently the pressure of theadjusting means is distributed over a small contact area, and theresulting'high specific pressure insures a good contact. Furthermore,even if an air film is formed between the contacting surfaces of theflexible electrode and the dielectric layer, its extent and averagethickness .variations are small and thus any variations thereof have buta negligible influence on the condenser. However, for larger capacitysettings the contacting surface of the flexible electrode increases,with a corresponding decrease of the specific pressure; this results ina poorer contact and produces an air film the variations of which,because of its greater extent and greater average thickness variations,have an important influence on the condenser capacity.

I have found that a trimmer condenser which at any capacity setting isfree from erratic capacity variations, can be obtained if in accordancewith the invention I use a construction in which the dielectric isdivided into two layers. The two dielectric layers, for example two micaplates, are each' provided with an outer adhering metallic coating,which coatings form the electrodes of the condenser.

To adjust the capacity of such a condenser various means may be used.For example, the two composite elements. (each comprising a dielectriclayer with an adhering metallic layer) may be adjustably spaced fromeach other by the interposition of a spring member of metal or ofinsulating material, or the composite elements maybe'biased externally,for example, by attaching to one of them a flexible spring member.

In the preferred embodiment the composite elements are adjustably biasedby an interposed metal spring which may be either a leaf spring or aspiral spring. It should be well understood that such metal springs,whatever their shape may be, do not electrically affect the condenser.

The condenser of the invention thus consists of two dielectric layers,for example of two mica plates interposed between two outer metalcoatings adhering to them.- As the metal coatings have a perfectmolecular contact with their respective mica plates, and the variableair gap is defined by the two mica plates, changes in pressure of theadjusting means merely vary the average thickness of this air gap and noerratic ca pacity variations occur.

It should be noted that irrespective of the capacity setting and theextent of pressure by the adjustment means, the mica layers because oftheir'smooth surface and identical hardness, neither scratch each othernor are they molecularly pressed into each other. It is importanthowever to avoid sharp edges of the spring member and it is preferablethat the contact area of the spring member with the mica plates remainssubstantially constant irrespective of the capacity setting.

In the drawing forming part of the specification:

Figure 1 is an enlarged schematic representation of a trimmer condenserin which the fixed plate is formed as a metallic layer coated on thedielectric layer;

Fig. 2 is a schematic representation of a trimmer condenser, comprisingtwocomposite elements each consisting of a dielectric layer and an outeradhering metal coating, and an interposed coil spring which serves asthe capacity adjusting means;

Fig. 3 is a schematic representation of a condenser similar to that ofFig, 2, but with an outer leaf spring serving as the adjusting means;

Fig. 4 is a schematic representation of a trimmer condenser similar tothat of Fig. 2, but with the composite elements consisting of dielectriclayers deposited on solid electrode plates;

Fig. 5 is an enlarged plan view of a condenser similar to that of Fig.2, but with a leaf spring used as the adjusting means;

Fig. 6 is a cross-sectional elevation along the line 6-6 of thecondenser of Fig. 5. Referring to the drawing, Fig. 1 representsschematically a trimmer condenser of the compression type in which ametallic layer l which flrmly adheres. to a dielectric layer 2,preferably of mica, serves as the fixed electrode of the condenser. Themetallic layer I may consist of a thin layer of silver, gold, platinum,zinc, copper, or the like deposited by various known methods on the micalayer 2. As a rule I prefer to use for the metallic layer silver anddeposit same by the process described in the copending application ofWilliam M. Allison and Mathew Nazzewski, Ser. No. 153,824, filed July15, 1937. This process also provides for a clean insulating border onthe mica, which eliminates any possibility of electrical leakage at theedges.

A flexible metallic plate 3 of phosphor bronze, or other similarresilient metal, serves as the adjustable electrode of the condenser.The capacity of the condenser may be varied by flexing the plate 3towards or away from the armature I by a suitable adjusting means shownas a screw 4. By distorting a portion of the plate, for example, byforming a pintle 5, plate 3 is mechanically biased to press against theadjustment screw 4.

As stated before, a condenser of a construction as shown in Fig. 1 willperform satisfactorily only for small percentage capacity settings.Howarvaoee ever, the condensers hereafter described in connection withFigs. 2 to 6 perform satisfactorily at any adjustment within theircapacity range.

The condenser of Fig. 2 comprises two dielectric layers 6 and 1?,preferably two mica plates, the outer faces of which are provided withadhering metallic layers 8 and 9 respectively, which form the electrodesof the condenser. The metallic layers 8 and 9 are preferably formed inthe same manner as has been stated in connection with layer l of Fig. 1.distanced from each other by an interposed helical metal spring I0,which for the adjustment of the capacity of the condenser is compressedby external adjusting means (not shown) to the desired extent.

The condenser of Fig. 3 is similar to that of Fig. 2, except that themechanical biasing means are formed by an external resilient metal platell attached to the armature 9 and provided with a distorted pintlesection l2. The capacity of the condenser is adjusted by varying theaverage distance between the composite elements, for example by theadjustment screw 4.

The condenser illustrated in Fig. 4 comprises two metallic plates l3 andI4 serving as the condenser electrodes, each plate being provided on itsinner surface with an adhering thin dielectric layer it and i6respectively. The layers 05 and it consist of a low-loss dielectricmaterial and may be applied to the plates by various wellknown methods,for example, they may be formed by painting or spraying on the metalplates l3 and ll a thin film of a solution of Victron or of a suitableresin, whereby after the coating, the solvent may be volatilized byheating the plates. A very good dielectric layer for this purpose can beobtained by cataphoretically depositing on the surface of the plates aflexible insulating material such as hard rubber. By curing, thedeposited insulating material forms a thin layer firmly adhering to theplate.

The condenser shown in Figs. 5 and 6 comprises a base 283 of aninsulating material having low dielectric losses such as Bakelite",porcelain or the like. The base 2% is provided on its top face with sidewalls 2l2l between which is disposed the condenser assembly, comprisingtwo mica plates 2d and 25, the respective outer faces of which areprovided with firmly adhering silver layers 22 and 23 respectively, thelayers 22 and 23 forming the condenser electrodes.

The silver layers are preferably deposited by the above-mentionedprocess whereby to prevent electrical leakage, the edges of the micaplates 2 3 and 25 are left unsilvered. The composite element composed ofthe mica plate 25 and its silver backing layer 23 is flat and is securedto the base 22 by means of a rivet 29 together with a plate 21interposed between the layer 23 and the base 20. The plate 21 ispreferably of soft copper or other highly conducting metal and serves asa low-resistant electrical connection for the electrode layer 23. Theplate 2i is provided with an extending lug portion 28 forming one of theoutside terminals of the condenser.

Interposed between the mica plates 24 and 25 and pressing against themica plate 24 is a curved rectangular spring plate 26 made of phosphorbronze or similar highly resilient material. On top of the compositeelement 22-24 is a curved resilient rectangular metallic plate 3i whichserves as a low resistant contact for the electrode layer 22. The plate3| is provided with an extending lug 36 which serves as the second ex-The mica plates 6 and l are ternal terminal of the condenser. The plate3i, and the composite element 22-26 are secured to the base 2@ by arivet 3d.

The base 2t is provided with a central hole 32 adapted to receive ascrew 33, which screw passes through corresponding apertures of theplate 2?; composite element 23-25, spring 26, composite element 22-22,and plate M, the respective apertures of these parts being sufficientlylarge to prevent contact between same and the screw. The screw 33 withits nut 33% disposed in an hexangular recess 35 of base 20, forms theadjustment means of the condenser.

Interposed between the head of the screw 33 and the plate 30 is aninsulating washer 31 of mica, porcelain, Bakelite, or the like.

As will be noted from the drawing, the composite element 2224 issufficiently flexible to assume under the pressure of curved springplate 26 substantially the same shape as has the curved plate 3i; thisholds for any position which the plates 3i and 26 may assume due to thediiierent settings of the adjustment screw 33. The arrangement alsoprovides for sufiicient elasticity of composite element 2224 to permit asmooth continuous variation of the capacity setting.

The side walls 2l2l of the base serve to prevent rotation of the partsabout the rivets 29 and 30.

I have found that the wearing qualities of the metallic layers 22 and 23are much improved by either providing on top of the same a'coating of ahard metal as copper electrolytically deposited thereon or by making themetallic layer as such a harder and. thicker coating.

The insulating base 22 can also be replaced by a metal base which may atthe same time serve as the conducting plate 2?], whereby no side wallsare required as the riveting of the condenser elements to a metal baseinsures them from displacement. In this case the rivet 32 is to beinsulated from the base.

I have found that condensers made in accordance with my invention, showremarkable stability in capacity, irrespective of the number of heatcycles or the temperature ranges to which they are subjected.

For instance, when condensers made in accordance with the invention areplaced in an electric oven and passed through three consecutive heatcycles, from 25 C. to 65 C. and back, no appreciable change in theircapacity can be detected, while condensers of the prior art subjected tothe same test, show permanent capacity changes of several percent,whereby the higher the initial capacity of the condenser, the greaterthe change. I have found that even the best of present day condensershave capacity drifts three or four times as great as the condensers ofmy invention.

Furthermore, the temperature coeflicient of capacity of the condensersof the invention because of their high stability can be made positive ornegative, or substantially zero, as it may be desired.

A further important advantage is that with trimmer condensers made inaccordance with the invention, maximum capacity settings can be obtainedwhich are about 50% greater than those of prior art condensers havingthe same dimensions and total thickness of the dielectric layer.

While I have described my invention in connection with specificembodiments and examples, I do not wish to be limited thereto,-butdesire the appended claims to be construed as broadly as permissible inview of the prior art.

What I claim is: 1. In an adjustable condenser, a condenser electrode, aflexible composite element consisting of a metal layer and a dielectriclayer, said layers being in molecular contact with each other, saidelement adapted to be flexed relative to the electrode to vary thecapacity of the condenser. 2. In an adjustable condenser, a condenserelectrode, a flexible composite element consisting of a thin dielectriclayer and a thin metallic layer disposed on one face of said dielectriclayer and in molecular contact therewith, said element adapted to beflexed relative to the electrode to vary the capacity of the condenser.

3. In an adjustable condenser, two mica plates and two metallic layersserving as electrodes, one metallic layer forming a strongly adheringouter coating on one mica plate and the other metallic layer forming asimilar layer on 'the'other mica plate, said mica plates facing eachother and means to vary the capacity of said condenser by flexing one ofsaid layers towards the other.

4. In a trimmer condenser, a stationary elec trode and a flexibleelectrode, and a dielectric layer interposed between said electrodes,said dielectric layer comprising a thin flexible mica plate, saidflexible electrode consistingof a metallic layer adhering to the outerrace of said plate, aresilient metallic backing member in electricalcontact with said flexible electrode, resilient distancing meanscontacting with the inner face of the mica plate, and adjusting means tovary the shape of th mica plate and its average distance from thestationary electrode, said mica plate assuming substantially the shapeof said backing member.

5. An adjustable condenser comprising a com posite element consisting ofa mica layer and a metallic layer adhering to the outer face thereof,said metallic layer forming the stationary electrode of the condenser, asecond composite element consisting of a flexible mica plate and ametallic coating adhering to the outer face thereof and forming theadjustable electrode of the condenser, a resilient metallic backingmember in electrical contact with said coating, a concave spring memberinterposed between the two mica plates, and adjustment means to vary theshape of the flexible mica plate and its average distance from thestationary electrode, said mica plate assuming substantially the shapeof said backing member.

ROBERT C. SPRAGUE.

